1. Technical Field
This technology pertains generally to organic light-emitting devices (OLEDs) and methods of fabrication, and more particularly to low cost methods for fabricating flexible, transparent devices with at least one nanotube-nanowire-polymer composite electrode producing OLEDs with enhanced emission efficiency.
2. Background Discussion
Organic light-emitting diodes (OLEDs) have emerged as an important technology for flat panel displays and solid state lighting. In addition to high efficiencies that can surpass that of fluorescent tubes, one of the most attractive advantages of the OLEDs have been their potential to be produced through solution-based processes at ambient temperature and pressure that are compatible with roll-to-roll manufacturing, large areas, and flexible form factors. However, some of the originally perceived major advantages of OLEDs have not been fully realized in commercial products.
Early attempts at producing flexible OLEDs used indium-doped tin oxide (ITO) as the conductive layer coated on glass or plastic substrate. However, the flexibility of these devices is limited by the glass substrate or by the ITO coating. In addition, flexible OLEDs based on ITO/plastic substrate have exhibited generally low performance and rather limited flexibility. High performance OLEDs that are generally fabricated on ITO/glass and are not flexible. Furthermore, the emission efficiency of OLEDs is limited due to waveguiding in the ITO layer as well as the substrate layer.
Conventional OLEDs also employ a cathode that is deposited by physical vapor deposition in a high vacuum which is costly. Vapor deposition methods such as plasma vapor deposition (PVD) or chemical deposition (CVD) schemes require complicated equipment and a substantial capital investment, greatly increasing the cost of the device fabrication process. In addition, the materials available for use in conventional vapor deposition processes are limited to mainly metal oxides or mixed metal oxides, such as indium-tin mixed oxide (ITO), antimony-tin mixed oxide (ATO), fluorine-doped tin oxide (FTO), and aluminum-doped zinc oxide (Al—ZO). Unfortunately electrodes produced from these metal oxide materials are found to be brittle and adhesion to the substrate is poor. Such electrodes do not survive repeated deformations.
Accordingly, there are several key challenges that have yet to be overcome for all-solution processing: 1) the limited selection of a transparent anode. ITO coating requires a high vacuum and high temperatures. While several alternative technologies have been studied, none of them can match the performance of ITO/glass. 2) The cathode generally uses low-work-function metals such as aluminum (Al) deposited by physical vapor deposition in high vacuum. 3) For efficient injections of charge carriers, the emissive organic layer has to be thinner than 100 nm, and multiple layers are preferred, which make all-solution processing rather difficult.
There is a need for organic light-emitting devices with improved efficiency as well as all-solution processing methods of manufacture of flexible, transparent devices that do not require costly vacuum deposition processes. The technology described below satisfies these needs as well as others and is generally an improvement over the art.